German preliminary published application DE 101 48 177 A1 discloses a valve train with a valve lift switchover for the gas-exchange valves. The valve train comprises cam pieces that are mounted non-rotatably and axially movably on a spline shaft and that have at least two adjacent cams with a different lift for each individual gas-exchange valve. Moreover, in order to move the cam pieces radially to the cam piece, movable housing-fixed actuators are provided that interact with movement grooves that are configured to be mirror-symmetrical and that intersect each other on the cam pieces, whereby each cam piece is provided with an actuator pin. For a reciprocal movement of the cam pieces, the actuator pin in question is brought into contact with a corresponding flank of the intersecting, and thus contiguous, movement grooves. This valve train is only suitable for a valve lift switchover between two adjacent cams. If more than two cams are to be switched over, a multiple arrangement of movement grooves and actuator pins is necessary. Moreover, since the movement grooves do not have a non-ambiguous course, switching inaccuracies can occur due to the actuator pins.
German preliminary published application DE 10 2007 056 692 A1 discloses a valve train device of an internal combustion engine with a valve lift switchover. Two switching sleeves that each have a link track and an actuator pin that can be brought into contact with the appertaining link track are provided for the valve train switchover of two axially movable cam elements, whereby a first switching sleeve causes the cam elements to move in a first switching direction, while a second switching sleeve causes the cam elements to move in a second switching direction. Accordingly, during a valve switchover, in the case of a first switching direction, the first switching sleeve is used to move the first cam element and to subsequently move the second cam element. In the case of a second switching direction, the second switching sleeve is used to move the first cam element and to subsequently move the second cam element. For this purpose, the link tracks of the switching sleeves are s-shaped and configured to be mirror-symmetrical to each other. This valve train is only suitable for a valve lift switchover between two adjacent cams. Furthermore, this valve train involves a great deal of production and assembly work as compared to other valve lift switchover systems.
German preliminary published application DE 10 2007 061 353 A1 discloses a valve train for gas-exchange valves of an internal combustion engine with a moveable cam carrier. The cam carrier is mounted on a camshaft so as to be non-rotatable but axially movable, and it has several cams at an axial distance that have different elevations and that are combined to form a cam group for the appertaining gas-exchange valve. An engagement element continuously engages with a continuous groove running along the circumference of the cam carrier, and this engagement element executes a lift in the axial direction corresponding to the course of the continuous groove. The engagement element can be locked in certain positions, as a result of which the cam carrier is forced to make an axial movement so that it switches over between the cams in order to drive a gas-exchange valve. However, the described system requires a great deal of installation space, meaning that it can only be used in internal combustion engines that have a sufficient distance between the cylinders. The engagement element has to be continuously carried along and there is a risk of greater wear and tear. Moreover, for each movable cam carrier, an actuator is needed in order to lock the engagement element, and this actuator has to be activated and then deactivated at the right point in time.
German preliminary published application DE 10 2009 039 733 A1 discloses a valve train for gas-exchange valves of an internal combustion engine in which a cam unit with several different cam tracks arranged adjacent to each other is mounted so as to be non-rotatable and axially movable on a camshaft. In order to switch over the lift of the gas-exchange valves, the cam unit can be moved axially by an adjustment means. The adjustment means comprises an adjustment element that is mounted so as to be movable inside the camshaft and that serves to transmit the adjustment movement to the cam unit. For this purpose, the circumference of the adjustment element has a track profile that cooperates with a coupling element, and moreover, the coupling element is connected to the cam unit by means of a recess in the camshaft. An axial movement of the cam unit on the camshaft is effectuated by a movement of the adjustment element relative to the camshaft. A variant embodiment describes that the adjustment element is rotatably mounted in the camshaft and that the track profile extends essentially in the circumferential direction of the adjustment element. The change between two cam tracks on the cam unit for the actuation of the gas-exchange valves is effectuated by an axial offset in the track profile that results from a rotation of the adjustment element relative to the camshaft. Due to a rotation relative to the camshaft, more effort is required in order to switch forward and backward between the different cam tracks by means of a reversal of the direction of rotation of the adjustment element. Moreover, high stresses arise as a result of the requisite axial rise in the track profile.
German patent DE 10 2011 101 868 B4 discloses a device for a valve lift switchover. For this purpose, a cam carrier is arranged axially movably on a camshaft, and different cam profiles are made on the cam carrier in order to establish an operative connection to an associated charge-exchange valve. In order to axially move the cam carrier, an adjustment shaft is provided parallel to the camshaft, and a first transmission element is arranged on the adjustment shaft so as to be non-rotatable and axially movable. The first transmission element has a guide track along which a housing-fixed first engagement element and additionally a second engagement element of a second transmission element run when the adjustment shaft is rotated. The second transmission element is connected to the cam carrier in such a way that the cam carrier is moved axially when the adjustment shaft is rotated. This device for the valve lift switchover can be used for more than two different cam profiles, whereby only a sequential switchover between the different cam profiles is possible.